Professor Sycamore and Alexa explore some ruins, where they discover a stone statue of a woman named Aila, and a legend emerges. A collection of Kalos records reveals that Aila's true love, Jan, attempted to vanquish the Destruction Pokémon Yveltal. When he failed, the land was drained of all life as Yveltal became a cocoon and turned Aila to stone.
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Jan continued his quest, seeking out the Life Pokémon Xerneas to replenish the barren land—although it couldn't save Aila. When Kalos seemed hopelessly out of balance, the Order Pokémon Zygarde appeared to restore it, and Jan remained at Aila’s side as the cycle of life and destruction continued.
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(Source: Bulbapedia)